This invention relates generally to sintering of ceramic and metal parts. More particularly, it relates to a method of sintering parts that preserves the alignment of complex structures and prevents separate part sections from fusing together during sintering. This method can be used to sinter pre-assembled mechanisms consisting of interlocking sections such that the sections do not fuse together during the sintering process and render the mechanism inoperable.
Parts can be made from a variety of ceramic and metallic materials using powder based processes. The powdered material is combined with a binder and formed into a shape. The binder serves to hold the powder particles together and gives the formed part sufficient mechanical strength that it will maintain its shape. The part is referred to as a xe2x80x9cgreenxe2x80x9d part. Green parts typically consist of approximately 50% by volume of powder particles, the rest being binder and air. Green parts typically have relatively low mechanical strength. After forming the green part, a burnout process, at temperatures typically up to about 600 C, is used to remove the binder material from the green part and begin fusing the powder particles together. Sintering is the final step where the powder particles are fused together at high temperature, typically at above 1500 C for ceramic materials. After sintering the parts are typically between 95 and 100% dense and are mechanically strong.
A typical green part has up to 50% by volume of air and binder. During sintering the part undergoes a significant amount of shrinkage and also becomes soft. Because the shrinkage may not be uniform and/or because the part may not be supported properly, the part may deform from its original shape or sag. Further, adjacent green parts may shift such that they make contact with one another and fuse together. Because of the shrinking, sagging and shifting, described above it is difficult to sinter complex green structures having more than one green part where the parts are in close proximity. Further, using conventional sintering processes it is extremely difficult to sinter complex green structures formed with independent interlocking and interlocking movable parts without the parts fusing together. This is particularly difficult for parts separated by small gaps.
Prior Art:
To help prevent the distortion of green structures during sintering, some processes use extended supports to hold up projecting regions of the structure. While extended supports, do reduce sagging of the structure during sintering, because the extended structures do not experience the same rate and amount of shrinkage as the green material, the shape of the structure may still be distorted during sintering. Block supports are also used, but block supports only support base portions of the green structure and do not provide uniform support to the entire green structure during sintering. Support structures with a combination of a base support and extended supports can be used to support complex green structures, but again the unequal shrinkage between the green structures and the supporting structures is still problematic for retaining the shapes of parts and alignments between multiple parts.
Powder beds have been used to provide more uniform support for complex green structures during sintering. When a powder bed is used, the green structure is buried or partially buried in a powder, which acts to support individual parts of the structure during sintering. Because the powder bed is not rigid it allows the parts to shrink during sintering while still providing support. Powder beds provide one of the best methods known in the prior art to support complex green structures during sintering and yet there are still several shortcomings.
Because the powder bed is compliant it will not prevent warpage or distortion during sintering. It primarily reduces sagging due to gravity by supporting the undersides of parts. Powder beds are also not suitable for use in sintering mechanisms for two main reasons. First, in cases where there are small gaps between mechanism sections, for example in joints, it is very difficult to manually align the sections such that they are not touching while also placing the powder around them. It is also very difficult to place powder into the narrow gaps to prevent contact between the sections. In many cases, because of the joint design, it will also not be possible to inspect the joint to ensure that the two sections are not touching and that there is powder between the sections. Second, because the powder bed is compliant it may allow mechanism sections to come into contact as a result of movement caused by shrinkage during the sintering process. If mechanism sections come into contact they may fuse together and render the mechanism inoperative.
With the current developments in rapid prototyping very complex green structures can be fabricated. These complex green structures may consist of a number of interlocking sections and may contain internal cavities and regions. When these complex green structures are sintered, it is extremely problematic to keep the individual sections aligned using methods described in the prior art. In view of the above mentioned shortcomings with the prior art methods, what is needed is a general method for sintering green parts which preserves the shape of the part and reduces sagging during sintering. Further, what is needed is a method for sintering complex green structures which preserves the alignment of the individual parts within the complex green structures.
It is a primary object of the present invention to provide support structures that support green parts during sintering which help preserve the shape of the part. It is a further object of the present invention to provide a method of sintering complex green structures with a plurality of green parts, which preserves alignment of the green parts during the sintering process.
In the method of the invention, the green parts or green structures and the support structures are preferably both made from the same green material, such as a ceramic or metallic powder with an appropriate binder. Because the green parts and the supports are made of the same material they exhibit similar degrees of shrinkage during sintering which helps to prevent shifting and sagging of the part during sintering.
For the fabrication of a structure, the green parts and the support structure are preferably formed in a single process, such that the green parts and the support structure are a monolithic piece of green material. The monolithic green structure is formed by any suitable method known in the art such as injection molding or Shape Deposition Manufacturing, whereby the structure is built-up by sequential deposition of layers.
The support structure preferably has a base to provide stabilization with elongated support fixtures extending from the base to the structure. After the green support structure and the green parts are sintered, the support structure is detached form the part by cutting or breaking away the elongated support fixture.
The method of the current invention is practically well suited to sintering complex structures having independently movable interlocking parts that are required to maintain precise alignment during sintering in order prevent them fusing together. For example, the method is well suited for making ceramic turbine structures with an outer rotor and an interlocking movable shaft, described below. When using the invention to sinter complex structures with interlocking parts, independent support fixtures are attached to each part of the interlocking structure to maintain alignment during sintering.